13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fracture mechanisms during intergranular hold time fatigue crack growth in Inconel 718 superalloy. Sten Johansson1,*, Leif Viskari2, Krystina Stiller3 Magnus Hörnqvist4 , Johan Moverare1 1 Department of Engineering Materials, Linköping University, Linköping 581 83, Sweden 2Chalmers University of Technology and SKF Sweden 3Chalmers University of Technology Sweden 4Chalmers University of Technology and GKN Aerospace Engine Systems Sweden * Corresponding author: Sten.Johansson@liu.se Abstract Ni-base superalloy IN718 is known to display time-dependent intergranular crack growth under dwell time mechanical loading at high temperature under atmospheric conditions. Oxygen has been pointed out as a cause of the intergranular damage causing embrittled crack growth during both cyclic and hold time loading. Investigation of the mechanisms responsible for the embrittlement should not only focus on the effect of environment but also on the combined action of fatigue, creep, temperature and time. In this work material from experiments with fatigue crack growth in combination with hold times of different length at different temperatures has been investigated. Fractographic studies and metallographic cross sections of fatigued specimens has been subjected to careful analysis using ECCI- imaging in order to shed light on the fracture mechanisms. The results show that the damage is caused by the influence of a combination of environment and severe local damage manifested as a transformation of the microstructure into sub cells, micro twins and recrystallised areas close to the crack tip. The damage mechanism is thus influenced by a combination of oxidation and severe local plastic deformation. Keywords Intergranular damage, Dwell time, ECCI, Fatigue, Fractography 1. Introduction Ni-base superalloys are widely used in high temperature and for applications like rotating discs in gas turbines where IN718 is a popular alloy due to relatively low price/performance ratio, good corrosion resistance and mechanical properties with excellent weldability. The high temperature performance is limited to 650°C and an increase in temperature needed to increase efficiency has created demands of increasing temperature in steps of 50°C. Since IN718 has a strengthening phase of DO22 based on Ni and Nb called γ ’’ the max temperature is 650°C while a similar precipitate based on Ni and Al (γ’) dissolves at a higher temperature. A newly developed γ’-former called Allvac718plus [1] is a replacement candidate for IN718. The Ni-base superalloys are during service subjected to a combination of static loading and fatigue loading controlled by stress as well as strain. In addition to that high temperature and environment will act to reduce service life. The growth of fatigue cracks will be affected by temperature and environment so that the mode of crack growth is shifted from transgranular to intergranular. This embrittlement effect that most superalloys have in common [2] has been analysed in a great number of publications [3-8]and reviews [9-11] [12]. The fact that not only fatigue fracture crack growth is influenced by, frequency, temperature and environment but also growth of a static crack in conjunction with fatigue has been subject to analysis [13, 14]. The conclusions have so far been that the significance of the effect is minor under present service conditions but with increasing service temperature and load the effect might be a serious matter for the future. The aim of this work is to study the effect of growth of fatigue cracks at high temperature in IN718 and Allvac718Plus in order to better understand the mechanisms behind the growth of fatigue
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